The detection of fluorescence signals is currently employed for a variety of different purposes.
For example, fluorescence techniques are used in the field of non-destructive testing (NDT). Such techniques can be applied to detection of small discontinuities such as inclusions, seams, shrink cracks, tears, laps, flakes, welding defects, grinding cracks, quenching cracks and fatigue cracks. In such techniques, a fluorescent material is applied to an item of interest in such a way that it preferentially accumulates in cracks, surface flaws and other defects on the item of interest. The item is then illuminated with electromagnetic radiation having a suitable wavelength so as to excite the material and cause the fluorescent material to fluoresce—this provides a user with a visual indication of the cracks/surface flaws/other defects on the item. Such defects might otherwise be difficult or even impossible to detect during a simple visual inspection of the item. Exemplarily known techniques employing this principle include Fluorescent Magnetic Particle Inspection (FMPI) and Penetrant Inspection for surface cracks and surface flaws in non-ferromagnetic and both surface and sub-surface cracks and flaws in ferromagnetic materials respectively. NDT inspection of articles via the detection of fluorescence in the above manner is of great importance across many industrial sectors and is used for example in forgings, foundry, fabrication, atomic energy, aerospace, ship building, railways, and automotive, applications. It is an inspection method which is prescribed by various standards bodies (for example, British Standards BS4489 and BS667, Civil Aviation Authority Air Worthiness Notice 95 and Rolls Royce Standard RPS702).
Other examples applications are for inspection of product within the electronics and semiconductor industries where a common use of penetrant fluorescent dyes is within pressured vessels, pressurised to approximately 7 bar, which contain signal diodes. Under these high pressures, fluorescent penetrant dye is forced into cracks which occur typically at the metal electrode to glass junction interface. Currently, manual inspection and detection of defective components exposed to penetrant dye under pressure is performed in very low light conditions.
As another example, fluorescence techniques are also used in medical contexts, in applications in fields such as Fluorescence Image Guided Surgery (FIGS) and Enzyme Linked Immunosorbent Assay (ELISA) techniques. The former is a surgical technique which detects fluorescently labelled structures such as cancers and tumours during surgery. The latter is a commonly used analytic biochemistry assay technique used in medicine, plant pathology and also as a quality control measure in various industries that produces fluorescence to detect the presence of a substance, usually an antigen or other substance. The techniques has further applications, for example in detecting auto-fluorescing biofilms and other pathogens, cell counting, microscopy and identification of chemical species.
However, in the known fluorescence techniques discussed above, the amount of fluorescence produced is typically low. This leads to poor signal to noise ratios when the technique is used in the presence of background illumination and makes it difficult for cameras and the human eye (for example that of the person conducting the test) to discern the fluorescence signal over the background illumination. For example, in NDT techniques such as FMPI and Penetrant Inspection, an inspector may be unable to see fluorescence associated with a surface defect when viewing an item in ambient daylight. In the case of FIGS, the low levels of fluorescence often requires the operating theatre lights to be dimmed or even switched off. In the case of ELISA, poor signal to noise ratios can limit the environments in which it can be deployed and the accuracy of quantitative analyses.
It is known in the art to improve signal to noise ratios of such fluorescence techniques by minimising the level of background illumination. For example, in surgical techniques, the ambient lighting is reduced to a level suitable for detecting fluorescence as noted above. However, such reductions in ambient lighting may be inappropriate, since higher lighting levels may be required for normal surgical purposes. For NDT applications ambient lighting must be low enough to comply with relevant requirements, (as specified in ASTM E709, for example) which often limits its deployment to being used indoors or under specially constructed covered awnings and tents whilst working outdoors in daylight conditions (direct sunlight). Limiting the use of NDT fluorescence techniques to indoor environments or requiring the installation of purpose-built tents or other structures requires additional time and manpower to perform the testing, thereby increasing costs and the time associated with performing such tests.
In many NDT applications, the sample of interest may be moving during testing. For example, in the case of the inspection of objects such as castings and forged parts, the objects can be on a fast-moving conveyor belt. Similarly, the apparatus used for testing may itself be moving (for example, if the apparatus is being hand-held). This poses a problem for conventional machine-vision camera systems for detecting fluorescence, in that they are often unable to capture clear images of the object of interest because there is an inherent delay between the capture of successive images. It is therefore possible for a defect to go undetected, either because the resulting image is too unclear to recognise the defect, or because the area of the object comprising the defect passed the field of view of the conventional machine-vision camera during the system's “dead time” between captured image frames. Conventional machine vision cameras are known to have different frame rates. However even with the faster available frame rates, such systems typically include a substantial “dead time” between captured image frames. Accordingly, such systems do not solve the problem above in relation to moving images.
Accordingly, there is a need to provide a more effective means to improve the signal to noise ratio of such fluorescence techniques in situations where background illumination is present. In addition, there is a need to provide such a means that is also capable of accurately detecting fluorescence in moving objects.